Question

Find the 11th term of the arithmetic sequence.
a3 = 20.5 and a8 = 13, a11 = ______.

Answer 1

ok uhmmm first we gata use the formula
which i believe is a + (n - 1)d
so if a3 = 20.5
then,
a3 = a + (n - 1)d
n= 3
20.5 = a + (3 - 1)d ----- 1
20.5 = a + 2d ----- 1

then,
a8 = a + (n - 1)d
n= 8
13 = a + (8 - 1)d -------- 2
13 = a + 7d --------- 2

So we are meant to find what a11 is
n = 11, a =?
a11 = a + (11-1)d
a11 = a - 10d -------3

Well we have a simultaneous eqn already

So,
20.5 = a + 2d
13 = a + 7d

now lets solve for a in eqn 1

a = 20.5 - 2d -------- 4
substitute in eqn 2

so, 13 = (20.5 - 2d) + 7d
13 = 20.5 + 5d
5d = 13 - 20.5
5d = -7.5
d = -7.5/5

So sub in eqn 1 or 2 to get a
so using eqn 2 a = 13 - 7d
a = 13 - 7(-7.5/5)
a = 23.5

so we have a and d
hence a11 = a +(n - 1)d
if a = 23.5 , n = 11
then a11 = 23.5 + (11 -1)(-7.5/ 5)
a11 = 23.5 + (10)(-1.5)

Answer 2

a 11 = 8.5

Answer 3

a + 2d = 20.5
a + 8d = 13

-6d = 7.5

-d = 1.25

d = -1.25

a - 2.5 = 20.5

a = 20.5 + 2.5

a = 23

a + d = 23 - 1.25 = 21.75 is a2

23 +2(-1.25) = 20.5 is a3

23 + 8(-1.25) = 13 is a8

23 + 11(-1.25) = 9.25 is a11


Please remember to click good answer

Answer 4

your answer is wrong cornzyblack

Answer 5

and your formula is also incorrect

Answer 6

its called openstudy

Answer 7

when did you realise?

Answer 8

mr. lig i tink u gat beef with me u want me to show u a flaw in your work, check line 2, u'll see u did not subtract to get a + 7d = a8, so back off. almost everytym i come here some1 must piss me off

Answer 9

a + 7d = 13

your write i made a typo

but atleast i can admit my mistake admit your wrong and your answer is also wrong as well as your method and we have nothing further to talk about lol

Answer 10

am definitely sure am not wrong

Answer 11

lol are you saying im wrong then?

Answer 12

Well, Idk if cornzy is right or not, but lilg, your answer was incorrect. I used yours and got that question wrong.

Answer 13

yes i'm working it out now because of my typo i have to do it again

Answer 14

see lilg if am wrong i would like someone to tell me not u from u, k. Beause you just pointed tha finger at me when u did rubbish

Answer 15

well done cornzyblack you got it right

Answer 16

a11 = 8.5